The invention relates to methods and apparatus for determining the location of a signal source. The determination of the source location may include determining the direction and range to the source from an antenna which receives the source signal.
Presently, there are methods for determining direction and range to a signal source. In one such method, disclosed for example in U.S. Pat. No. 4,339,755 to Wright, a pair of antennas spaced apart by a known distance may be utilized to receive a signal from the source. The antennas are arranged such that a given feature of the signal is received by one antenna before the other antenna due to the different travel distances for the signal to each antenna. The temporal relationship of the signal received by the two antennas may be used to determine the difference (DELTA) in travel distance to one antenna relative to the other.
A right triangle may then be formed where the two antennas are apices, and the third apex is located at a distance DELTA from the further antenna along the line between the antenna and the source. The distance between the antennas corresponds to one side of the triangle while the distance DELTA corresponds to a second side. A trigonometric equation relates the sides and angles of the right triangle and the equation may be solved for the direction to the source from one antenna.
Triangulation techniques may be utilized to determine the range to the source using two or more direction to source determinations.
A disadvantage of the prior art direction and range to source methods is that they require the use of two or more antennas and receivers. As these systems are often installed on an aircraft, the added weight and space allocation of the additional antennas may pose a problem, particularly in smaller aircraft where weight and space considerations are critical.
The direction to source resolution of prior art systems is limited by the separation distance of the antennas on the aircraft. The separation of the antennas is limited by the dimensions of the aircraft and is typically very small in relation to the distance to the source. The relatively small separation of the antennas means that even a very slight error in the detection of the source signal may cause a large error in the direction to source determination.
Embodiments of the present invention also utilize trigonometric relations to solve for the direction and range to a signal source. Unlike the prior art however, a single antenna and receiver are employed to receive the signal emitted from the source and make the necessary detections.
The received signal carries precisely repetitive symbols which enables the detection of symbols and the prediction of expected future occurrences of symbols. The single antenna system is used to detect the actual occurrence of signal symbols as the antenna is moved through several antenna positions. A prediction is made of the expected symbol occurrences at the antenna positions based upon the known repetition rate of the symbols at a first antenna position. A comparison of the actual and expected symbol occurrences for each antenna position indicates the change in the distance between each antenna position and the source relative to the distance between the first antenna position and the source. Accordingly, the distance between each antenna position and the source may be described as a function of the distance between the first antenna position and the source.
The determination of the functional relationship of the distances between each of the antenna positions and the source enable the solution of trigonometric equations for the direction and range to the source from any of the antenna positions.
It is accordingly an object of the invention to provide a method and apparatus which may require only a single antenna for determination of the direction and range to a signal source from a position of the antenna.
It is another object of the invention to provide a method and apparatus which may rely upon a signal having periodic symbols in order to predict the expected future occurrences of the signal symbols and so that a determination of the change in the distance between an antenna position and the source relative to another antenna position can be made by comparing expected and actual symbol occurrences.
It is yet another object of the invention to make a temporal comparison of expected and actual symbol occurrences.
It is still another object of the invention to make a phase comparison of expected and actual symbol occurrences.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.